1. Field of the Invention
The present invention relates to a coherent detection laser radar system for detecting objects
2. Description of the Related Art
There are two basic types of laser radar systems for detecting objects; those based on direct detection of optical radiation and those based on coherent detection of optical radiation. In a laser radar system based on direct detection of optical radiation, a beam of radiation is transmitted to an object, scattered off of the object, and the scattered or reflected portion is detected. In a laser radar system based on coherent detection, radiation scattered off of the object (return radiation or return beam), as well as radiation remaining within the laser radar system (a local oscillator beam) are detected.
FIGS. 1a, 1b and 1c show conventional coherent detection laser radar systems. FIG. 1a shows a heterodyne coherent detection laser radar detecting system, and FIGS. 1b and 1c show homodyne coherent detection laser radar systems.
In a heterodyne coherent detection laser radar system, the return radiation is mixed with radiation from a second laser. As shown in FIG. 1a, a laser beam generated by a first laser 102, having a frequency f.sub.1, is transmitted through a beam splitter 120 to a scanning device 121, and scanned across the object to be detected. As the beam is scattered off the object, part of the scattered radiation (the return radiation or return beam) is reflected back into the system (designated in the figures by a dashed line with arrows), reflected by beam splitter 120 to beam combiner 122 and directed into an optical detector 130. At the same time, a second laser 112 (a local oscillator) generates a second laser beam (the local oscillator beam) having a frequency f.sub.2, which is transmitted to beam combiner 122 and mixed with the return radiation at the optical detector 130.
Optical detector 130 converts the optical energy of the mixed beams into an electrical signal which can be processed and displayed by components (not shown) according to conventional techniques.
In conventional homodyne laser radar detection systems, only one laser is utilized. As shown in FIG. 1b, laser 102 outputs a laser beam having a frequency f.sub.1 which is split by a beam splitter 170 into two beams 172 and 174. Beam 172 is transmitted through beam splitter 120, scanned across the object to be detected, reflected back into the system, and input into optical detector 130 in a manner similar to that described above with respect to the heterodyne system of FIG. 1a. Beam 174 is reflected off mirror 176 into an optical frequency shifter 180 where the frequency f.sub.1 of the beam 174 is shifted by .DELTA.f to a new frequency f.sub.2. The output beam of the optical frequency shifter is combined with the return beam 178 by beam combiner 122 and input to the optical detector 130.
As is known, laser radar detecting systems using coherent optical detection require the simultaneous detection of two optical beams having different frequencies. The coherent lasers used in conventional laser radar systems have finite linewidths which translate to finite bandwidth signals at the output of detector 130. Further, system backscatter that is mixed with the local oscillator beam makes it difficult to detect signals corresponding to a slowly moving object to be imaged. For example, FIG. 6a shows the output of detector 130 of FIG. 1c in the frequency domain. This signal has a finite -3dB bandwidth determined by the coherence linewidth of the laser and the sample time of the signal. A CO.sub.2 gas laser can have a linewidth on the order of 75 kHz. The peak frequency of this signal corresponds to the frequency of the local oscillator beam f.sub.lo minus the frequency of the return radiation f.sub.r. If the object to be imaged is at rest, f.sub.lo -f.sub.r =0 and the peak frequency is at DC. A similar spike results from the system of FIG. 1a, but at a frequency equal to f.sub.2 -f.sub.1. This spike also occurs in the system of FIG. 1b, but at a frequency corresponding to the shift imparted by optical frequency shifter 180.
As shown in FIG. 6b, a signal corresponding to an object to be detected must fall outside the frequency range of the signal components corresponding to the local oscillator beam and internal backscatter to avoid being swamped out by these large signal components. Because the signal from the object to be detected is derived from the transmitted beam, the transmitted beam must be sufficiently frequency shifted to discriminate the signal corresponding to the object to be detected.
Conventionally, this frequency shift is obtained using modulating devices, shown as dashed boxes 190 and 192 in FIGS. 1a1c. The modulating devices 192 may comprise, for example, acousto-optic or electro-optic modulators alone or in combination with polarizers and/or birefringent retardation plates, etc., to modulate the frequency of or pulse the transmitted radiation and/or the local oscillator beam. Control circuitry 190 provides the necessary signals to drive the modulators 192 in accordance with conventional techniques In FIGS. 1a-1c, the control circuitry 190 directly modulates the output of the laser 102 and/or laser 112. Alternatively, modulating device 192 can be positioned after laser 102 to modulate the laser beam output by the laser in accordance with signals from control circuitry 190. In FIG. 1b, modulating device 190 can also be used to directly modulate the output of frequency shifting device 180.
Although the modulating means 190 and 192 allow the conventional systems of FIGS. 1a-1c to detect objects at rest, significant disadvantages result from the use of modulators 190 and 192. They are expensive and require complicated control circuitry to synchronize the movement of the scanning means 121 with the modulation of the laser beam to be transmitted. This significantly decreases the signal processing speed of conventional systems.
Alternatively, the systems of FIGS. 1a-1c can be used without the modulation devices 190 and 192, but only to detect moving objects. This implementation is based on the principles of the Doppler effect which imparts the necessary frequency shift to the transmitted radiation if the object is moving with sufficient speed. This implementation is disadvantageous in that the system cannot be used to detect objects at rest or objects moving at a speed which does not impart a sufficient Doppler frequency shift to the transmitted radiation to allow proper detection of the object to be detected